Shock isolator for rotary drill string



1966 J. H. WIGGINS, JR 3,263,446

SHOCK ISOLATOR FOR ROTARY DRILL STRING Filed Aug. 20, 1963 John H. Wiggins Jr.

INVENTOR.

BY @8, km

ATT RNEY United States Patent 3,263,446 SHOCK ISOLATOR FOR ROTARY DRILL STRING John H. Wiggins, .lra, Tulsa, Olden, assignor, by mesne assignments, to Esso Production Research Company, a corporation of Delaware Filed Aug. 20, 1963, Ser. No. 303,327 2 Claims. (El. 64-45) This invention relates generally to the rotary drilling of boreholes in the earth, and more particularly to an improved rotary drill string assembly. Specifically, the invention relates to shock isolator means for improving the drilling action of conventional rotary bits and for prolonging the life of the drill string and drilling rig.

Prior investigations have repeatedly established the occurrence of adverse vibrations in the operation of a rotary drill string, and the consequent need for incorporating a resilient shock-absorbing sub-assembly therein. Various explanations have been advanced for such vibra tions in the drill string. As can readily be seen, the bit and drill collars are elevated a short distance as each successive bit tooth or row of teeth contact the hole bottom. This alone is one source of vibration. Moreover, it has been found that each such elevation is not always followed immediately by a corresponding descent of the drill column in time for the next tooth to achieve proper contact with the hole bottom. The bit and collar may even be thrown sharply upward for a substantial time interval entirely free of engagement with the rock, following a particularly pronounced irregularity in the cutting action. Various influences retard the fall of the drill assembly following such a disturbance. The bit and I collars are quite massive and because of their high inertia are not easily returned to the hole bottom in time for the next successive cutter tooth engagement.

Regardless of their origin, drill string shock and vibration should be controlled in order to optimize bit performance, minimize hole deviation problems, decrease wear and fatigue rates throughout the drill string and surface rig equipment, and thereby reduce drilling costs. Current practices of attempting vibration control at the rig floor usually result in a departure from optimum bit performance. For example, rotary speeds are sometimes altered in order to reduce vibrations, even with the knowledge that drilling rate is thereby decreased.

Accordingly, one principal object of the invention is to provide means for continuously maintaining a sustained average loading of the drill string and bit assembly whereby. adverse vibrations are minimized and a more eflicient engagement of the bit cutters with the hole bottom is accomplished.

A further object of the invention is to provide an integral, fluid transmissive, tubular, shock-isolating subassembly for efficient control and transmission of longitudinal and rotational forces Within a rotary drill string.

In accordance with one embodiment of the present invention a novel shock isolating sub-assembly is incorporated in a drill string immediately above the drill bit. The vertical flexibility of the device allows compressional shocks and vibrations to be filtered thereby increasing the life of bit teeth and bearings.

In addition the device may be utilized at other locations within the drill string. For example, it may be placed near the upper end of the drill collar assembly at a position with respect to the bit which takes advantage of the natural resonant frequency of a drill collar assembly in response to compressional waves generated by dynamic vertical motions of the bit.

It is a further object of the invention to provide means of minimizing dynamic oscillations in a drill string by incorporating therein a device in which no sliding fluid 3,263,446 Patented August 2, 1966 sealsare required. A shock isolating sub-assembly is provided in which no sliding motion or other interaction of sleeves or other parts is present to cause binding and eventual failure due to friction wear.

The device of the present invention is simple and inexpensive to manufacture, and at the same time provides the necessary resilient response to vertical shock and also maintaining excellent continuity of torsional force transmission.

In essence, the invention is a tubular shock-isolating tool, at least a portion of which consists of bellows-like convolutions. The ends of the tool are adapted for connection between two joints of drill pipe or collar, or between the lowermost drill collar and a conventional bit assembly.

FIGURE 1 is a longitudinal sectional view of the bellows-shaped shock isolator, shown in a drill string adjacent the bit.

FIGURE 2 is a longitudinal sectional view of an alternate embodiment of the invention, showing an internal safety element for carrying excess tensile loads.

FIGURE 3 is a fragmentary longitudinal sectional view of an embodiment wherein the bellows-like convolutions are rounded.

Referring to FIGURE 1, a preferred embodiment of the invention comprises shock isolator 11 provided with threaded pin 12 and thread box 13 for connection in a drill string between collar 14 and bit assembly 15. The tool has a circular transverse cross-section, and consists essentially of a series of bellows-like convolutions 16, formed by a corresponding series of alternate internal and external circular grooves or channels 17, preferably filled with rubber or its equivalent, as shown. A tubular sleeve 18 surrounds the bellows-shaped interval of the tool as a safety feature to protect it from excess tensile loads, such as may be encountered in lifting the drill string. In this event, expansion of the bellows will cause flange 19 to engage shoulder 20, thereby diverting a fraction of such tensile loads to sleeve 18. Although the sleeve is attached by means of weld 21 in the illustrated embodiment, it is within the scope of the invention to provide a shoulder and flange or other means of holding sleeve 18 in place.

The embodiment of FIGURE 2 includes a bellows-like series of convolutions 31, which are essentially the same as in the embodiment of FIGURE 1. Internal tubular element 32 is a safety feature for avoiding excess tensile loads in the bellows structure. Thus, flange 33 rests upon rubber cushion 34, which in turn is supported by shoulder 35. Similarly, when high tensile loads are encountered, the bellows structure expands, causing flange 36 to engage shoulder 37, thereby diverting excess tensile stress to tube 32. Spring 38 acts to stabilize tube 32 during normal operation, when little or no tensile stress is encountered.

FIGURE 3 illustrates an alternate embodiment, wherein convolutions 41 are rounded to provide uniform wall thickness and reduce erosion.

The simplest operable embodiment of the invention consists of a thick-Walled cylinder having screw-threaded ends and a plurality of alternate internal and external circular grooves or channels spaced along a portion thereof to provide a longitudinal bellows-like flexibility substantially greater than is characteristic of the original cylinder. The grooved or convoluted interval of the cylinder need not extend substantially the entire length of the tool, as shown in the drawings. Thus, a limited shock isolation effect is obtained from only a single convolution. However, at least three convolutions are generally required to provide substantial flexibility, and preferably at least five.

Steel .is generally preferred as the construction material because of its superior strength, durability, and relatively low cost. However, other metals and alloys are contemplated for use, including particularly aluminum and beryllium-copper alloys.

In the drawings, rubber or its equivalent is shown molded into the grooves or channels of each convolution. The rubber need not bear a significant fraction of the loads imposed on the tool, although it can be designed to do so. It functions mainly to reduce erosion caused by mud flow, and to protect highly stressed areas from corrosion.

The safety sleeve 18 of FIGURE 1 serves no purpose during compressional loading of the bellows, and the same is true of tube 32 in FIGURE 2. These safety elements are useful primarily when the bit or drill string below the shock isolator becomes stuck, and as a consequence high tensile loads are placed on the isolator. The bellows-like convolutions could thus be inadvertently strained beyond their elastic limit, except for the safety element.

While grooves or channels 17 of the illustrated embodiments are uniformly spaced along the length of the tool, and each has the same dimensions, it is within the scope of the invention to provide numerous variations of these embodiments. It is essential, however, that the grooves have a depth greater than one-half the difference between the outside and inside radii of the tool. This follows from the fact that each of convolutions 16 necessarily includes a transverse section between adjacent grooves. It is preferred that each successive internal groove be spaced from each adjacent external groove by a distance no greater than four groove widths.

The spring rate of -a circular bellows, in pounds of force per inch of deflection, varies inversely with the number of convolutions, inversely with the square of the outside diameter, directly with approximately the cube and directly with the modulus of elasticity of the construction material. The bellows-shaped portion of the present invention preferably has a wall thickness of at least A inch, an outside diameter of at least 3 inches, and a spring rate no greater than 10 pounds per inch per convolution. The root diameter, which is equal to the inside diameter plus twice the wall thickness, is at least one inch.

As readily appreciated by one skilled in the art, residual stresses are an advantage in loaded members if they oppose the working stresses. In the case of bellows springs, longer life and increases up to 100 percent, as determined by tests (30 percent is common in coil springs), in maximum allowable working stress can be obtained by controlling or generating residual stresses in accordance with this general rule. To effect this control, stress relief and cold setting or presetting are usually performed.

A convenient method of constructing the shock isolator of the invention is simply to machine a thick walled cylinder internally and externally to provide grooves or channels 17. For example, a short length of thick walled tubing having an OD. of 7" and an ID. of 5", made of S.A.E. 4140 steel having a tensile strength of 80,000 lbs. per square inch, was machined externally to a depth of inch and internally to a depth of M2 inch to provide a bellows shape having -a root diameter of 5% in., and a wall thickness of A; in. It was preset, tested, and found to withstand a static vertical compressional load of 22,800 pounds at the elastic limit. It has a spring rate of 720,000 pounds per in. per convolution, and a maximum deflection of 0.0317 in. per convolution.

The shock isolator of the present invention possesses many advantages over prior tools designed for vibration control. Its essentially one-piece construction eliminates any need for sliding parts or fluid seals. It is conveniently designed to have a large internal bore, thus providing low hydraulic pressure loss for drilling fluid circulation. It isolates flexural shocks and vibrations as well as dynamic axial loads. Pressure differences between the interior and exterior of the tool act to provide a hydraulic load carrying capacity, in addition to mechanical support. The bellows shape stresses the metal in many ways thereby utilizing many strength properties of the material.

While various embodiments have been specifically described, other variations and modifications will occur to those skilled in the art, without departing from the proper scope and spirit of the invention.

What is claimed is:

1. A shock isolator coupling comprising an essentially tubular element adapted for connection in a rotary drill string, at least a portion of said element consisting of bellows-like internal and external convolutions at least partially filled with a resilient material; a perimetric shoulder on said tubular element intermediate one end thereof and said convolutions, said shoulder facing said one end; and a sleeve surrounding said convolutions and attached to said tubular element intermediate said convolutions and the other end thereof, said sleeve extending beyond said shoulder and comprising a flange positioned to engage said shoulder in the event a substantial tensile load is placed on said convolutions.

2. A shock isolator coupling comprising an essentially tubular element adapted for connection in a rotary drill string, at least a portion of said element consisting of bellows-like internal and external convolutions at least partially filled with a resilient material; a shoulder within the bore of said tubular element intermediate each end thereof and said convolutions; and a second tubular element within the first named tubular element having a flange at each end thereof to engage each said shoulder in the event a substantial tensile load is applied to said convolutions.

References Cited by the Examiner MILTON KAUFMAN, Primary Examiner.

ROBERT C. RIORDON, BROUGHTON G. DURHAM,

Examiners. H. C. COE, Assistant Examiner. 

1. A SHOCK ISOLATOR COUPLING COMPRISING AN ESSENTIALLY TUBULAR ELEMENT ADAPTED FOR CONNECTION IN A ROTARY DRILL STRING, AT LEAST A PORTION OF SAID ELEMENT CONSISTING OF BELLOWS-LIKE INTERNAL AND EXTERNAL CONVOLUTIONS AT LEAST PARTIALLY FILLED WITH A RESLILENT MATERIAL; A PERIMETRIC SHOULDER ON SAID TUBULAR ELEMENT INTERMEDIATE ONE END THEREOF AND SAID CONVOLUTIONS, SAID SHOULDER FACING SAID ONE END; AND A SLEEVE SURROUNDING SAID CONVOLUTIONS AND ATTACHED TO SAID TUBULAR ELEMENT INTERMEDIATE SAID CONVOLUTIONS AND THE OTHER END THEREOF, SAID SLEEVE EXTENDING 